The object of this project is to elucidate how general anesthetic agents and muscle relaxants alter respiratory mechanics so as to adversely affect pulmonary gas exchange. Human studies have been limited by ethical considerations and remain conjectural as to the mechanism for the fall in functional residual capacity and lung compliance found with anesthesia in horizontal postures. To permit in depth analysis of this anesthesia-induced altered respiratory function, this project has been designed to study these parameters in chronic trained tracheostomized dogs. The initial thrust of this project will be to determine the pressure-volume curves of the lung and chest wall, the lung and chest wall compliance, the pulmonary resistance, the intrapulmonary inspired gas distribution (ventilation per unit lung volume) and the chest wall (rib cage, diaphragm and abdomen) shape and motion in the awake, anesthetized and anesthetized-paralyzed and mechanically ventilated dogs whilst they are lying in the prone, supine and lateral decubitus positions. The dose response curves of several anesthetic agents including halothane, enflurane, methoxyflurane and thiopental will be so studied. Lung volumes will be assessed by body plethysmography, pressures with appropriate transducers, intrapulmonary inspired gas distribution by the xenon 133 method and nitrogen clearance studies, and chest wall shape and motion defined using magnetometer transducers and biplane x-ray analysis. If a particular anesthetic regimen and posture causes a fall in functional residual capacity and lung compliance (as occurs in humans), the possible mechanisms responsible will be thoroughly investigated. These include changes in cast wall shape and motion which will be defined by using muscle relaxants and selectively paralyzing the diaphragm and thoracoabdominal wall. Other possibilities which will be assessed include airway closure, altered intrapulmonary inspired gas distribution, reduced surfactant formation and changes in the lung tissue. If no anesthesia technique is found to cause a fall in functional residual capacity and lung compliance in dogs in horizontal postures, the causes of this important and extremely relevant species difference will be researched by analysis of the above variables.